Epidemiologic data support the association between high intake of vegetables and
fruits and low risk of chronic disease. There are several biologically plausible
reasons why consumption of vegetables and fruit might slow or prevent the onset
of chronic diseases. Vegetables and fruit are rich sources of a variety of
nutrients, including vitamins, trace minerals, and dietary fiber, and many other
classes of biologically active compounds. These phytochemicals can have
complementary and overlapping mechanisms of action, including modulation of
detoxification enzymes,
stimulation of the immune system,
reduction of platelet aggregation, modulation of cholesterol synthesis and
hormone metabolism, reduction of
blood pressure, and antioxidant,
antibacterial, and antiviral effects. Although these effects have been examined
primarily in animal and cell-culture models, experimental dietary studies in
humans have also shown the capacity of vegetables and fruit and their
constituents to modulate some of these potential disease-preventive mechanisms.
The human studies have relied on intermediate endpoints related to disease risk.
Design methodologies used include multiple-arm trials, randomized crossover
studies, and more compromised designs such as nonrandomized crossovers and pre-
and posttreatment analyses. Length of treatment ranged from a single dose to
years depending on the mechanism of interest. Stringency of dietary control
varied from addition of supplements to a habitual diet to provision of all food
for the duration of a treatment. Rigorously conducted experimental dietary
studies in humans are an important link between population- and laboratory-based
research.